1. Technical Field
The present application relates generally to a spread spectrum coding in a CDMA (Code Division Multiple Access) mobile communications system and, in particular, to a device and method for generating a spread spectrum signal using a pseudo-orthogonal code.
2. Description of the Related Art
In a CDMA mobile communications system, communication is conducted within a given frequency bandwidth that is shared by multiple users who are assigned differential codes. A data transmission rate for a user is generally very low relative to the frequency bandwidth. Spread spectrum coding is used to transmit a low-rate data with the high-rate frequency bandwidth, as well as for discriminating between users. Specifically, low-rate data bit sequences are spread with a high-rate spreading code so as to be transmitted/received in the given frequency bandwidth.
In a CDMA mobile communications system, an orthogonal code spreading scheme using Walsh codes is typically employed for user discrimination and spectrum spreading. Ideally, the orthogonality of the Walsh codes enables users or channels to be discriminated without interference.
Referring now to FIG. 1, a block diagram illustrates a conventional spread spectrum signal generating device using Walsh codes. A signal mapper 111 converts 0s and 1s of an input data bit sequence to +1s and xe2x88x921s, respectively. An orthogonal code spreading and PN (Pseudo random Noise) masking unit 117 spreads the signal values +1s and xe2x88x921s at a high rate. Specifically, the orthogonal code spreading and PN masking unit 117 orthogonally spreads the signal received from the signal mapper 111 with an assigned Walsh code Wi and then performs a PN masking on the spread signal using PN codes, PNi and PNq, to discriminate base stations or users. The PN-masked signals, Xi and Xq, are baseband-pass-filtered by baseband filter 119 and converted to a radio signal by frequency shifter 121.
Referring now to FIGS. 2A, 2B, and 2C, various embodiments are illustrated of the orthogonal code spreading and PN masking unit 117 of FIG. 1. FIG. 2A illustrates one embodiment of the orthogonal code spreading and PN masking unit 117 for a conventional IS-95 CDMA mobile communications system. In order to perform orthogonal spreading, a multiplier 211 multiplies an input signal of +1 or xe2x88x921 by an assigned Walsh code Wi. The spread signal is a complex signal which is separated into a real and an imaginary component. The real and imaginary components are applied to multipliers 212 and 213, respectively. The multipliers 212 and 213 multiply the respective spread signals by a pair of PN codes, PNi and PNq, to perform PN masking.
FIG. 2B illustrates another embodiment of the orthogonal code spreading and PN masking unit 117 which doubles the number of available Walsh codes. In FIG. 2B, a serial-to-parallel converter 231 separately outputs odd-numbered and even-numbered signals of +1 or xe2x88x921. Then, multipliers 222 and 223 multiply the odd-numbered signal and the even-numbered signal by the Walsh code Wi, respectively. For PN masking, a multiplier 224 multiplies the output of the multiplier 222 by a PN code, PNi, and a multiplier 225 multiplies the output of the multiplier 223 by a PN code, PNq. Since the transmission rate of a +1 or xe2x88x921 signal in the directions of real and imaginary parts is half of the input transmission rate utilizing this method, the Walsh code length should be doubled. Thus, the number of available Walsh codes is virtually increased by factor of two.
FIG. 2C is another embodiment of the orthogonal code spreading and PN masking unit 117 of FIG. 1, which, as in FIG. 2, utilizes double the number of available Walsh codes (as compared to the embodiment of FIG. 1). In addition, PN masking is performed through complex spreading to thereby make the signal strengths of the real and imaginary component equal. In FIG. 2C, the serial-to-parallel converter 231 separately outputs odd-numbered and even-numbered signals of +1s or xe2x88x921s. Then multipliers 232 and 233 multiply the odd-numbered signal and the even-numbered signal by the Walsh code Wi, respectively, and output signals di and dq. A complex multiplier 234 multiplies di and dq by PNi and PNq, respectively, and outputs PN-masked signals, Xi and Xq. The complex multiplier 234 operates in accordance with the following formula:
(Xi+jXq)=(di+jdq)*(Pni+jPNq)xe2x80x83xe2x80x83(1)
The embodiment of FIG. 2C enables a signal to be recovered without interference because the Walsh code used for generating the spread spectrum signal exhibits a correlation value of 0 with respect to another Walsh code under an ideal condition (i.e., single path propagation).
Referring now to the graphs of FIGS. 3A and 3B, correlation characteristics of Walsh codes are illustrated. FIG. 3A illustrates the relationship between signal delay and auto-correlation, and FIG. 3B illustrates the relationship between signal delay and cross-correlation. In the case of auto-correlation as shown in FIG. 3A, a spread spectrum signal generated in the orthogonal code spreading and PN masking unit 117 of FIGS. 2A, 2B, and 2C is recovered with a strength equal to the length N of a Walsh code in code synchronization. A spread spectrum signal generated in the orthogonal code spreading and PN masking unit 117 of FIGS. 2A, 2B, and 2C will have a correlation value 1 (but not 0) when code misalignment occurs by a time delay of one or more chips. In the case of cross-correlation as shown in FIG. 3B, there is no interference when two Walsh codes are synchronized. But when code misalignment occurs by one or more chips, a 1-interference signal appears (i.e., an interference signal having a strength of 1/N relative to that of the original signal). Consequently, the influence of the interference signal is inversely proportional to the length N of the Walsh code. If a signal is received from at least two paths, and a delay of one or more chips exists between the paths, the orthogonality of the Walsh code is lost and an interference is generated due to the delayed signal.
The issue is how to define a delay time of one or more chips in the above situation. To provide a high rate data service, the frequency bandwidth should be increased, which implies that the time duration of one chip becomes shorter. The duration of one chip Tc is equal to the inverse of the available frequency bandwidth:
Tc=1/BW
Therefore, as BW doubles, Tc decreases by half. Hence, a signal which is transmitted from a single path in a voice only service may exhibit a multipath propagation characteristic (i.e., a time elapse for at least one chip duration when an available frequency bandwidth is widened for a high-speed data service) which can I(Lz result in the loss of orthogonality of the Walsh code.
Therefore, it is an object of the present invention to provide a pseudo-orthogonal code generating device and method for providing a high-quality, high-speed data service over a CDMA mobile communications network.
It is another object to provide an encoding device and method, which can maintain the orthogonality of a signal transmitted on a multipath propagation channel by compensating for the delay time of the signal.
It is still another object to provide an encoding device and method which can prevent loss of the orthogonality of a spreading code caused by a multipath signal component by spreading data with a multipath resistant pseudo orthogonal code (MRPOC).
In one aspect, a method for generating a pseudo-orthogonal code for orthogonally spreading channel data in a CDMA mobile communications system includes the steps of selecting M orthogonal codes from N orthogonal codes to form a pseudo orthogonal code, sequentially interlacing the elements of the selected M orthogonal codes to generate the pseudo-orthogonal code as a sequence of Mxc3x97N elements.
In another aspect, a device for orthogonally spreading channel data in a CDMA mobile communications system comprises a pseudo-orthogonal code generator having a table for storing M orthogonal codes which are selected from N orthogonal codes to form pseudo-orthogonal codes, in the form of index pairs, and generating a pseudo-orthogonal code as a sequence of Mxc3x97N elements by sequentially interlacing the elements of the M orthogonal codes in an index pair corresponding to an input code index, the device also including a multiplexer for multiplexing input channel data to M-branch parallel data, a plurality of spreaders connected to each of the plurality of M-branches for spreading the M-branch data with M corresponding orthogonal codes by multiplication, and a demultiplexer for demultiplexing the parallel spread data to serial data.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments, which is to be read in connection with the accompanying drawings.